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Questions and Answers

What type of pressure does a Sylphon bellows measure?

Atmospheric pressure with no reference

Gauge pressure relative to atmospheric pressure

Differential pressure between two sources

Absolute pressure (psia) with a vacuum reference (correct)

What is the primary function of temperature compensation in pressure measuring instruments?

To ensure that external temperature changes do not affect pressure readings (correct)

To increase the accuracy of pressure readouts in extreme temperatures

To maintain a constant temperature within instruments

To calibrate instruments for different types of gases

Which statement about bimetallic strips is true?

They remain straight regardless of temperature changes.

They consist of two metals with the same temperature expansion coefficient.

The differing expansion coefficients cause the strip to curl when temperature changes. (correct)

They are used solely for measuring pressure variations.

When considering pressure measurement, what does psig stand for?

Pound per square inch gauge Signup and view all the answers

What is the Critical Mach Number (Mcrit) defined as?

The Mach number at which supersonic airflow first occurs on an aircraft. Signup and view all the answers

What impact can temperature have on an aneroid capsule in pressure measurement?

Cold temperatures make it less malleable, affecting its readings. Signup and view all the answers

What is the typical range for the Maximum Operating Mach Number (Mmo) in commercial jet aircraft?

M.8 to M.87 Signup and view all the answers

In pressure measurement, what does differential pressure reference?

The difference measured against a vacuum Signup and view all the answers

What is the primary function of a Machmeter?

To compute the ratio of airspeed to the speed of sound at altitude. Signup and view all the answers

Which mechanism in the Machmeter measures airspeed?

A conventional airspeed indicator measuring pressure difference. Signup and view all the answers

Why is it important to determine the Maximum Operating Mach Number (Mmo) for subsonic aircraft?

To avoid the risk of losing control due to shock wave disruption. Signup and view all the answers

What role does the aneroid capsule play in a Machmeter?

It provides altitude information by sensing static pressure. Signup and view all the answers

What feature distinguishes a non-sensitive altimeter from a sensitive altimeter?

It makes one complete revolution for every 10,000 feet in altitude. Signup and view all the answers

In what scenario is a cabin altimeter specifically used?

To indicate the height of the cabin above sea level. Signup and view all the answers

What is the range of measurement for sensitive altimeters?

35,000 to 50,000 feet or more. Signup and view all the answers

What compensatory feature does a sensitive altimeter have for temperature?

A bimetallic device. Signup and view all the answers

How many equally spaced marks are on the dial face of a sensitive altimeter?

10 marks. Signup and view all the answers

What is the primary purpose of the knob on the front of a sensitive altimeter?

To operate the mechanism that compensates for non-standard atmospheric pressure. Signup and view all the answers

Which components are part of the internal mechanism of a sensitive altimeter?

Three diaphragms in series. Signup and view all the answers

What is the function of the larger pointer on the sensitive altimeter dial?

To represent a 100-foot change in altitude. Signup and view all the answers

What is the primary purpose of a sensitive altimeter in aviation?

To maintain safe vertical separation between aircraft and terrain Signup and view all the answers

Which Q code is used to set the altimeter to read zero feet when on the ground at an airfield?

QFE Signup and view all the answers

Why is QNH pressure setting important during enroute navigation?

It helps determine terrain clearance and requires updates based on region Signup and view all the answers

What does the QNE code refer to in aviation?

Barometric setting of 1013.25 hPa when above transition altitude Signup and view all the answers

How does atmospheric pressure information conveyed by air traffic control benefit pilots?

It provides confidence in maintaining vertical separation Signup and view all the answers

What does the QNH setting do when landing and taking off?

Makes the altimeter reflect airfield elevation above sea level Signup and view all the answers

At what point must QNE be applied in aviation?

Above the transition altitude Signup and view all the answers

What must pilots ensure about altimeter settings within a specific area?

All altimeters must be set to a local QNH value for safety Signup and view all the answers

What does the term 'transition altitude' refer to in aviation?

The altitude where flight levels are defined Signup and view all the answers

Study Notes

Pressure Measurement with Bellows

Bellows measure high, low, and differential pressures.
Differential Bellows: Consist of two bellows from different pressure sources causing movement based on pressure difference.
Sylphon Bellows: One bellows is pressurized, the other evacuated; measures absolute pressure (psia) and references a vacuum.
Manifold Pressure Gauge: Example of Sylphon-type bellows by using an evacuated bellows to reference pressure against a vacuum.

Temperature Compensation in Instruments

Instrument readings are influenced by temperature changes, affecting malleability and pressure measurements.
Temperature errors can arise significantly at sea level and with altitude changes; instruments need to compensate for temperature fluctuations.

Bimetallic Strips

Composed of two metals with different coefficients of thermal expansion (e.g., brass and invar).
Heating causes the strip to curl towards the metal with a lower expansion rate (invar), while cooling leads to the opposite curl.
This curling motion is converted to rotary motion to drive a pointer on a scale.

Altimeter Types

Non-sensitive Altimeter: Utilized primarily in smaller aircraft, has a single pointer that completes one revolution for every 10,000 ft change in altitude.
Sensitive Altimeter: Contains multiple pointers or a pointer with a drum counter, designed to detect small pressure changes, covering altitude ranges of 35,000 to 50,000 feet.
Cabin Altimeter: Indicates cabin pressure, displaying height above sea level, crucial for pressurized aircraft.

Altimeter Q Code Settings

QFE: Sets altimeter to zero feet at the ground level of an airfield, regardless of airfield elevation.
QNH: Adjusts altimeter to show airfield elevation on takeoff and landing, critical for terrain clearance.
QNE: Applies to altimeter settings at 1013.25 hPa; relevant above transition altitude where flight levels are used.

Critical Mach Number and Maximum Operating Mach Number

Critical Mach Number (Mcrit): The point where supersonic airflow first occurs on an aircraft's airframe, typically around M.73 for commercial jets.
Maximum Operating Mach Number (Mmo): Determines safety margins to avoid control loss at high altitudes; usually ranges from M.8 to M.87 for commercial aircraft.

Machmeter Functionality

Combines airspeed and altitude to compute Mach ratio, follows differential pressure principles.
Adjusts airspeed based on static pressure readings from an aneroid capsule; compensates for temperature deviations aligned with ISA chart.

Effects of System Blockages on Instruments

Static Line Blockage: Can be caused by water ingress, plumbing kinks, or insect obstructions; leads to instrument failures and incorrect readings.
Consequences of Blockage: Affects altimeter responses, leading to no reading or sluggish performance; airspeed indicators behave based on altitude changes, complicating readings during a blockage.
Partial Blockage Effects: Results in sluggish altimeter responses, degraded vertical speed indications, and erroneous airspeed readings depending on altitude changes.
Alternate Static Port: Can restore accurate readings if blockage occurs in standard static lines.

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Description

Explore the concepts of pressure measurement using different bellows, including differential and Sylphon bellows. Understand the role of temperature compensation in instrument readings and the function of bimetallic strips in temperature measurement. This quiz covers essential principles for accurate pressure and temperature measurement in various applications.